So, I read up about stale_while_revalidate

# https://requests-cache.readthedocs.io/en/stable/user_guide/expiration.html#asynchronous-revalidation
stale_while_revalidate=True,

And at some point got pool size warnings:

WARNING  Connection pool is full, discarding connection: 192-168-x-y.[redacted].plex.direct. Connection pool size: 10  
WARNING  Connection pool is full, discarding connection: 192-168-x-y.[redacted].plex.direct. Connection pool size: 10  
WARNING  Connection pool is full, discarding connection: 192-168-x-y.[redacted].plex.direct. Connection pool size: 10

• Update http cache rules to be more effective Taxel/PlexTraktSync#1870 (comment)

so, I dug up some code:

• requests-cache/requests_cache/session.py

  Lines 295 to 299 in 2c143a3

  	def _resend_async(self, *args, **kwargs):
  	"""Send a non-blocking request to refresh a cached response"""
  	logger.debug('Using stale response while revalidating')
  	thread = Thread(target=self._send_and_cache, args=args, kwargs=kwargs)
  	thread.start()

and that code immediately raised me questions:

1. there's no control of how many threads can be created
2. there's no method to wait for the background threads to be finished